Thunderbird 9 .wav file won't play even with test button

Several TB versions ago, I had the sound working fine, but noticed today that it was not still working. No idea how long it has been dysfunctional, since I'm usually listening to music. It's a weird problem.

I have two old .wav files that I kept from my Win98, and I've had them in the same folder since I got Win7 on a new PC two years ago. That folder was not in a 'library', but it worked fine. Both files will play with 'Windows Media' when on the 'Choose Sound' screen (Tools Options General...select file, click arrow at right side), and the first one (call it file 'Car.wav') will play using the 'Play' button after selecting/opening it, but the other (Ohgod.wav), the one I want to continue to use, will no longer 'play' using that button, nor does it play when getting a new email.

I thought it might have something to do with not being in a 'library', so added the folder to 'Music'. No change.

I have just updated to TB 9, and that didn't fix it, either.
Any ideas would be appreciated.

Thanks, Roderunner, but that still won't make that file work correctly. Other files do work correctly. In "Sound", the "Test" button won't even play that file. Windows Media Player has no problem with the file, however. TB will play other sound files, but not this one particular one.

I've seen reports from a couple years ago, in my searches, where others had a similar problem, but it seems they were always "all" .wav files that wouldn't play, and usually on some Linix OS, and TB 3.x.x. So, I tried to find this exact file on the net again, hoping a fresh copy would help, but no luck finding it.

Did that, but no luck. After changing to that file for email notification, the "Test" button did nothing, and the Apply button was still greyed out. Strange!
I'm thinking now, of getting a hex editor and changing some little something in that file. From what I've read on the net, in years-old posts, there's some kind of "script" that plays wav files in TB, and the error they were getting had something to do with .......no data....... . Maybe unzipping it will change something??? I'm assuming that you do not use TB, right?

EDIT: My mistake - the Apply button was NOT still greyed out; that part (selection and apply) works fine.